The present invention relates to a method of and a system for detecting a misfire in a spark-ignition internal combustion engine.
Some modern automotive internal combustion engines are equipped with catalytic converters for re-oxidizing combustible residues in the exhaust gases discharged from the power cylinders of the engines. The catalysts incorporated in the converters are thus heated to considerably high temperatures throughout the operation of the engines and, for this reason, one of the most important design considerations in using such converters is to protect the catalysts from the attacks of heat. The temperatures in the exhaust gases rise to extremely high levels when misfires take place in the power cylinders of the engines and sometimes result in burning of the catalysts. Measures must therefore be taken to detect misfiring conditions of the engines and to eliminate the causes inviting the misfires in the engine cylinders. Various methods have thus far been proposed to detect the misfiring conditions of automotive internal combustion engines for the purpose of protecting the catalytic converters from the heat resulting from misfires in the engines.
One of such methods is to monitor the waveform of the current in the primary or secondary winding of the ignition coil provided in the spark ignition system of an engine. Such a method is, however, not fully acceptable because of the fact that a misfire in a spark-ignition internal combustion engine can be invited not only due to malfunction of the ignition system but by reason of an improperly proportioned air-fuel mixture delivered to the power cylinders of the engine.
Another prior-art method of detecting a misfire in an internal combustion engine is to measure the temperatures of the exhaust gases discharged from the individual power cylinders of the engine and to compare the detected exhaust gas temperatures with each other for detecting an unusual temperature level resulting from a misfiring condition. This method involves a drawback in that extremely complicated and costly mechanical and electrical arrangements are required to achieve satisfactory reliability.
A third method of detecting a misfiring condition is to monitor the concentration of air in the exhaust gases discharged from the engine cylinders for detecting failure of combustion in any of the cylinders from an unusual increase in the concentration of air in the exhaust gases containing an air-fuel mixture which has not been fired and combusted. The concentration of air in the exhaust gases is detected by the use of an oxygen sensor which is mounted within the exhaust system for producing an electrical output signal which varies with the concentration of air in the exhaust gases passed through the sensor. As is well known in the art, such as from U.S. Pat. No. 3,827,237, the output signal of such an oxygen sensor is substantially constant except when the concentration of oxygen contained in the exhaust gases assumes a certain value (which usually corresponds to the stoichiometric air-to-fuel ratio of the mixture produced in the mixture supply system of an engine). The oxygen sensor is thus unable to continuously and faithfully follow the variation in the concentration of air in the exhaust gases in an analog fashion and, for this reason, can not distinguish a condition in which a misfire has been brought about in the engine from a condition in which an overlean mixture has been combusted in the engine.